1. Technical Field
Various embodiments generally relate to a semiconductor device and an operating method thereof. Particularly, the various to embodiments relate to a semiconductor device capable of reading correct data even though a drift phenomenon occurs in a memory cell and an operating method thereof.
2. Related Art
In a memory device, there may occur a drift phenomenon. A drift phenomenon is where information (for example, a resistance value) of a memory cell is changed according to the passage of time. Such a drift phenomenon is particularly problematic in a phase change memory device (PCRAM).
FIG. 1 is a graph illustrating a change in the resistance of a PCRAM cell due to a drift phenomenon.
In the illustrated graph, the PCRAM cell is a multilevel cell capable of writing 2 bits and stores information of four states level 0 (00), level 1 (01), level 2 (10), and level 3 (11). A resistance value corresponding to each state is the largest in the case of level 0 and is the smallest in the case of level 3.
When the drift phenomenon occurs, the resistance value of the cell is changed according to the passage of time. Therefore, when a threshold value used at the time point, at which data is written, is applied to the time point at which the data is read, an error may occur. However, since it is not possible to know the degree in which the drift phenomenon progresses at the time point at which the data is read, it is difficult to find an exact threshold value.